The present invention relates to sensors for detecting the presence of selected components in a gaseous mixture. More specifically, the present invention relates to gas sensors that detect changes in the resistivity of thin film sensing elements caused by adsorption of gas molecules by the sensing elements.
Gas sensors are useful for safety monitoring, process monitoring and control, evaluation of gaseous product quality, environmental control, and so forth. For example, mercury vapor and hydrogen sulfide gas sensors are used in a variety of industrial hygiene and process control applications.
Typically, miniature fuel cells, known as electro-chemical sensors, are used to detect hydrogen sulfide gas. Such electro-chemical sensors are known to have poor zero stability, a short life span, are temperature sensitive, and have problems with loss of sensitivity and calibration errors caused by interfering gasses. In addition, attempts to apply electro-chemical sensors to the detection of selected components at parts-per-billion (ppb) levels have not succeeded because of an excessively long time required for response, typically tens to hundreds of minutes. This excessive response time is undesirably slow for process control applications.
Optical gas sensors depend on measuring the transmission of light at a different wavelength for each gas. The particular wavelength identifies the gas and the amount of light absorbed by the gas determines the gas concentration. Conventional optical gas sensors are large, complicated, expensive, and can encompass a cabinet full of several discrete components which are usually hand-selected and hand-assembled. Attempts have been made to build the optical components, i.e., emitter, filter, and detector, on a single silicon chip. While the silicon chip based optical gas sensors are an improvement over prior art optical gas sensors in terms of size, they still suffer from fragility, excessive cost, poor sensitivity, and instability.
Thin film gas sensors have also been developed to detect a selected component in a composite gas. A thin film gas sensor is formed of a suitable semiconductor material whose electrical resistivity changes in response to the adsorption of the selected component. One such thin film gas sensor includes a gold thin metal film layer deposited on a ceramic substrate. The resistivity of the gold changes in response to the adsorption of mercury or hydrogen sulfide. The electrical resistance of the gold film exposed to the gas is measured and provides a basis for determining the concentration of the selected component.
Adsorptive thin film gas sensors are typically regenerated after adsorbing a sufficient amount of the selected component to trigger an indication circuit. Regeneration of the thin film involves heating the thin film to liberate the molecules of the selected component adsorbed by the thin film layer, i.e., the gold film, to prepare the gas sensor for a new cycle of gas detection and measurement. Depending upon the type of molecules adsorbed, the regeneration temperature can exceed 250xc2x0 C. In prior art devices, the thin film layer is commonly used in both the sensing role and as a heater conductor for regeneration. Thin film gas sensors in which the thin film layer is used in both the sensing and heating roles are referred to hereinafter as combined sensor/heater thin film gas sensors.
Combined sensor/heater thin film gas sensors have been moderately successful in that they are typically more sensitive and more resistant to interference than electro-chemical and optical gas sensors. Unfortunately, combined sensor/heater thin film gas sensors suffer from a short life span. A principal failure mechanism involves the electromigration of the gold metal in the sensing film.
Electromigration is mass transport due to momentum exchange between conducting electrons and diffusing metal atoms. The result of electromigration is that metal atoms move from the thin gold film into the dividing layers on a chip. If electromigration occurs to a great degree, and enough metal atoms move into the dividing layers, the thin gold film may become too thin, resulting in failure of the gas sensor.
Electromigration is characteristic of metals at very high current density and temperatures of 100xc2x0 C. or more. Accordingly, electromigration is exacerbated when the thin film layer is used in both the sensing role and as a heater conductor. Consequently, catastrophic failure of the combined sensor/heater thin film gas sensor occurs after a relatively small number of cycles of sensing and regeneration due to the high regeneration temperature.
In addition to the problem of electromigration, combined sensor/heater thin film gas sensors suffer from a lower than desired sensitivity. The sensitivity of a combined sensor/heater thin film gas sensor is governed by its geometric design. In particular, the life span of such a sensor is inversely proportional to the width of the thin gold film. The aspect ratio of a thin film gas sensor is the length of the sensor trace divided by the width of the sensor trace. In order to tolerate the high regeneration temperatures, the aspect ratio must be low, i.e., a thick width relative to the length. A low aspect ratio limits the sensitivity of the combined sensor/heater thin film gas sensor to parts-per-million levels, rather than ppb levels.
Yet another problem with combined sensor/heater thin film gas sensors is that the resistance of the sensor/heater trace is high. Therefore, a high voltage (approximately 60-100 volts) is needed to regenerate the sensor. Consequently, the combined sensor/heater thin film gas sensors are often limited to use in areas where 120 VAC or suitable power generators are available.
Some prior art thin film gas sensors circumvent the aforementioned problems found with combined sensor/heater thin film gas sensors by utilizing external heating elements to heat the thin metal film to the regeneration temperature. Unfortunately, such external heating elements can be difficult to manufacture and to calibrate for specific sensor applications. Moreover, the amount of heat generated by such a heating element may vary over the surface of the sensing layer. Uneven heating is undesirable because it can cause insufficient or inconsistent regeneration.
Yet other prior art thin film gas sensors have been fabricated to include an integrally formed heater element. One such thin film gas sensor includes a silicon substrate and a silicon nitride membrane supported by the substrate. A thin gold sensor trace and a thin gold reference trace are deposited on the silicon nitride membrane and progress in a substantially parallel spaced relationship relative to one another. Air cavities are formed in the substrate under the silicon nitride membrane, such that the membrane forms a number of platforms suspended above the cavities. The sensor and reference traces are located on each platform. A heater element is supported on the platforms above the upper surface of the silicon nitride membrane, but below the sensor element. The air cavities prevent heat produced in the heater element from escaping into the substrate. In other words, the air cavities act as a barrier so that the heat is directed to the sensor trace for efficient regeneration of the sensor trace.
The use of a separate embedded heater mitigates the problem of electromigration as discussed above, and enables a relatively even distribution of heat to the upper surface of the membrane. However, problems associated with this design include complexity and excessive cost of manufacture, as well as, fragility. In particular, the thin silicon nitride membrane is easily fractured during manufacturing and handling.
Accordingly, it is an advantage of the present invention that an improved thin film gas sensor for detecting the presence of a selected component is provided.
It is another advantage of the present invention that a thin film gas sensor is provided for detecting very low concentrations of a selected component.
Another advantage of the present invention is that a thin film gas sensor is provided that is capable of substantially even heating during regeneration.
Yet another advantage of the present invention is that a thin film gas sensor is provided that is robust, and is readily and cost effectively manufactured.
The above and other advantages of the present invention are carried out in one form by a device for detecting a presence of a selected component in a gas. The device includes a substrate having a first surface and a second surface, the substrate exhibiting a low thermal resistance. A heater element is disposed on the first surface. A sensor element and a reference element are located on the second surface. The sensor element is configured to adsorb molecules of the selected component, and the reference element is configured to adsorb the molecules of the selected component at a substantially lower rate than the sensor element.